This invention relates to position and orientation measuring systems (POS) which determine remote object position and orientation and, more particularly, to an electromagnetic POS having a source which produces a plurality of electromagnetic fields which are distinguishable from one another, a remote sensor which senses the generated electromagnetic fields and a processor which processes the output of the sensor into remote object position and orientation relative to a reference coordinate frame of the source. More particularly, the invention relates to an improved source for an electromagnetic position and orientation measuring system.
Remote object position and orientation measuring systems employing electromagnetic coupling are disclosed in U.S. Pat. No. 3,868,565 issued to Jack Kuipers for an OBJECT TRACKING AND ORIENTATION DETERMINING MEANS, SYSTEM AND PROCESS and U.S. Pat. No. 4,054,881 issued to Frederick Raab for a REMOTE OBJECT POSITION LOCATOR. Such systems traditionally have a source assembly that includes a plurality, typically three, of concentrically positioned, orthogonal field-generating antennas for generating a plurality of electromagnetic fields. Signals are applied to the field-generating antennas that are multiplexed so that the resulting electromagnetic fields are distinguishable from one another. Located at the remote object is a sensor having a plurality, also typically three, of concentrically positioned, orthogonal receiving antennas for receiving the electromagnetic fields generated by the transmitting antenna and producing signals corresponding to the received electromagnetic fields. A processor resolves the signals produced by the receiving antennas into remote object position and orientation in the reference coordinate frame of the source.
One application for such position and orientation measuring system (POS) is as a three-dimensional digitizer for determining and digitizing the coordinates of a three-dimensional object. In such a digitizer, the source may be embedded in a table having an integral work surface, or digitizing tablet, upon which the object to be digitized is placed. A hand-held stylus is used for tracing and determining points of interest on the surface of the object. The remote sensor is disposed in the stylus and the stylus is provided with a projection for defining a point of contact between the stylus and the surface of the object of interest. Since the point of contact of the stylus and the remote sensor are provided with a known relationship, the coordinates of the surface of the three-dimensional object touched by the stylus may be determined from the position and orientation of the sensor in the coordinate frame of the work surface.
One difficulty with a digitizer utilizing an electromagnetic POS is that the work surface, or digitizing tablet, cannot be located on a table including a substantial amount of electrically conductive or ferromagnetic metal. The electromagnetic fields generated by the source of the apparatus induce eddy currents in the metal. The eddy currents produce an electromagnetic field which distorts the field generated by the source, inducing error in the measured fields and, hence, the calculated position and orientation of the remote object. Various methods exist to map distortion and to compensate for the distortion in the POS processor. However, if the distortion is too severe, such mapping may not adequately compensate for the distortion.
It may be desirable to be able to place a conventional digitizer source on an electrically conductive or ferromagnetic metal plate. Other applications for which it may be desirable to use an electromagnetic POS may also result in severe magnetic-field distortion with a conventional POS source. For example, it may be desirable to place the digitizer source on a metal table adjacent to a metal wall. It may also be desirable to position a POS source at the corner of a metal-walled rectangular room in order to locate one or more remote objects within the room. In all such applications, the ability to compensate magnetic-field distortions would be nil.